1. Field of the Invention
This invention relates to a process and to a composition of matter resulting from this process. More specifically, this invention is directed to processes for the preparation of spherical colloidal particles of rare earth (hydrous) oxides. The particles produced in accordance with these processes have a very narrow particles size distribution and well defined morphology. These particles have advantageous optical properties (i.e. fluorescence) and are, thus, useful in diagnostic applications in the optical separation of various constituents of complex fluids (i.e. blood, cerebrospinal fluid or urine).
2. Description of the Prior Art
The preparation of colloidal particles from organic substances has, until very recently, been a highly empirical "science". For the most part, the efficacy of such processes was quite subjective and generally the relative success or failure thereof required laborious trial and error in order to attain adequate process definition. More specifically, the efficacy of a particular technique, even if it were reproducible to a degree, rarely produced a consistently acceptable product. The inability to achieve reproducible results from such processes has, thus, led many to regard the synthesis of inorganic colloidal particles as largely the domain of the empiricist.
With the advent of more sophisticated analytical tools (i.e. electron microscopy), the fascination with inorganic colloidal particles, and more particularly, monodispersed inorganic colloidal particles, has been rekindled. The initial interest in such materials was primarily as a scientific curiosity, however, more recent developments have found them useful as supports for catalysts, in ceramics, pigments, films, recording media, coatings, in various diagnostic and therapeutic environments, as well as a myriad of other applications.
The term "monodispersed" as used in the discussion of the prior art and throughout the balance of this disclosure is intended as referring to a population of particulate materials having a narrow particles size distribution.
A survey of the various techniques for synthesis of monodispersed, inorganic colloidal particles has recently appeared in the technical literature, see Matijevic, E., "Monodispersed Colloids: Art and Science", Langmuir, Vol. 2, No. 1, pp. 12-20 (1986).
The procedures which have been developed by the inventor for synthesis of inorganic colloidal dispersions of narrow particle size distribution have been described in detail in a number of papers which have also appeared in the technical literature, see for example, Matijevic, E., Annu. Rev. Mater. Sci. (1985), 15, 483 and Matijevic, E., Acc. Chem. Res. (1981), 14, 22. Two of the procedures described in the above articles can be conveniently grouped into the following categories: (1) precipitation from homogeneous solution (i.e. forced hydrolysis, controlled release of anions and controlled release of cations); and (2) phase transformations. What is, however, to be appreciated is that each of the above procedures will have one or more shortcomings or advantages for synthesis of a specific colloidal material. Thus, the production of an acceptable product in accordance with each of the processes from the same starting materials is highly unpredictable. More specifically, in order to produce colloidal particles of specific characteristics, both of the above procedures may have to be attempted before one can be identified as potentially useful or efficacious. At that point, additional refinement will be required before an acceptable product is attainable.
In the procedures involving precipitation of inorganic compounds from homogeneous solutions, the precursors to the formation of the solid phase are, in most instances, one or more solute complexes. This procedure is, thus, based upon the control of kinetics of the complexation reaction in order to achieve a single burst of nuclei, which are then allowed to grow uniformly, resulting in particles of narrow size distribution. Where the constituent solutes are generated at the proper rate, their even distribution onto existing nuclei results in the least increase in total free energy of the dispersion, thus, controlling the growth of such particles by proper control of particle charge. Control of the charge in such particles is traditionally achieved by adjustment in pH or through the introduction of additives. In the absence of such control in charge, aggregation of such particles will result.
The phrase "forced hydrolysis" is used hereinafter to reference the process or ability of many hydrated metal ions (especially polyvalent metal cations) to readily deprotonate in aqueous solution at elevated temperatures. This characteristic can be used to advantage in the preparation of colloidal particles from such materials. Since the hydrolyzed species of these metal ions are intermediates to precipitation of the corresponding hydroxides, it is possible to generate uniform particles simply by heating metal salt solutions. In this forced hydrolysis procedure, the pH and the nature of the anions play a dominant role. In some instances, anions may simply affect particle morphology without being integrated within the solid phase, or can be incorporated within the solid phase as impurities into either an amorphous or crystalline solid. Lastly, these anions can through stoichiometric compounds, as in the case of alunites.
Because of the nature of the colloidal particles, and the various methods used in their preparation, their physical properties are often unpredictable. More specifically, the preparation of colloidal particles from rare earth oxides by traditional methods did not permit the attainment of particles of predictable morpology or uniform size.
The traditional procedures for the synthesis of rare earth oxides are both diverse and energy intensive (Kirk and Othmer, Encyclopedia of Chemical Technology, (2nd Ed), Vol. 17, 163). The so-called "dry" approach to such synthesis involves the initial formation of salts (i.e., hydroxide, carbonate, oxalate, nitrate, sulfate, etc.). These salts can be converted to the corresponding oxide by standard calcination techniques (at temperatures in excess of 850.degree. C.). These salts are, thus, decomposed to the corresponding oxides which are essentially insoluble in aqueous media.
The rare earth oxides produced in the above manner have had rather limited applications, and then primarily in industrial environments. Rare earth compounds, including rare earth oxides, have been mainly used in glass manufacturing and polishing, arc carbons, catalysts, lighter flints, and in ceramic applications.
The adaptation of colloidal materials to biological environment introduces a unique set of variables. For examples, if a colloidal material is to be used in a fluid environment, its ability to form stable dispersions can be critical. In the event the colloidal material is to be used as an indicator or a label, the photo-optical or magnetic properties may be of paramount importance. Colloidal particles are attractive for biological applications because they are relatively inert and can be produced in quantity from readily available materials at relatively low cost. Unfortunately, the inability to prepare such materials in a reproducible manner, with predictable properties, has hindered their general acceptance. Accordingly, there is a continuing need to provide a cost efficient reproducible process for the synthesis of inert, inorganic colloidal particles.